The present invention relates generally to flexible and semi-rigid film composites used for primary and secondary packaging within the retail, liquid, baked goods, mixes, beverages, confectionary, frozen, dry shelf, diary, meats, seafood, anti-static, dissipative, snack, shipping, sack, and bagged goods packaging industries. The composites having a substantially mineral-based or ground calcium carbonate-containing layer(s) such that it is highly attractive, has excellent printability, acts a barrier material, is highly efficient and significantly less expensive to manufacture, is pliable, scuff resistant and environmentally friendly.
Printed and unprinted primary and secondary flexible and semi-rigid packaging materials are commonly used for packaging retail, industrial, food, and commercial products into bags, sacks, pouches, wrappers, and the like. Key performance attributes of these materials include substantial barrier protection, product protection and containment, preservation, shipping, storage, and dispensing applications. Existing embodiments include preformed flexible containers, generally enclosed on all but one side, which form openings that may or may not to be sealed after filling, normally constructed of any single ply flexible material, multiple independent layers, flexible layers, and laminated constructions. Other related art includes inner liners or bags used for packaging consumer, food, or industrial products. Glassine, greaseproof paper, waxed paper, or plastic films are frequently used for this purpose in order to create the required contact surface or to provide a suitable barrier. With greasy products, liners prevent the staining of the bag material. Other applications include anti-static and dissipative film structures designed to protect packaging contents from accumulating the potential to deliver damaging electrical discharges.
Considerations taken into account in the development of such packages and materials include the cost of resins and the cost to extrude, blow, or cast the resins into film or sheets. Further costs include lamination into multi-layer constructions. Finally, the cost to convert, print and shape the films and their printability are crucial considerations. Many resins and converted flexible films are available to the market. Structural designs are often driven by barrier requirements between the enclosed product and the surrounding environment. In packaging, the term “barrier characteristics” is most commonly used to describe the ability of a material to stop or retard the passage of atmospheric gases, filled gases, water vapor, and volatile flavor and aroma ingredients. Barrier materials may serve to exclude or retain such elements without or within the package. Often sufficient barrier qualities can be achieved in design, however, the unprinted base film or base stock, which is the untreated film web-stock to which print, coatings, laminations, and other processes will be applied, does not contain adequate printability or is prohibitively expensive.
Printability is a key attribute for packages targeting the retail or point-of-sale industries. Printability is the ability of a material to yield printed matter of good quality. Printability is judged by the print quality and uniformity of ink transfer, rate of ink wetting and drying, ink receptivity, compressibility, smoothness, opacity, color, resistance to picking, and similar factors. Printability is different than run-ability, which refers to the efficiency with which a substrate may be printed and handled at the press. Further, structural and printability factors influence the ability of the materials to be printed using specific printing equipment. It is generally preferred if a material can be printed on a variety of equipment, maximizing quality of print and minimizing cost of manufacture. Printing techniques include flexographic, roto-gravure, heat-set, heat transfer, offset, offset lithography, non-contact laser, ink-jet, ultra-violet, hot stamp, screen, silk-screen.
Another key factor is process-ability, which is the ease with which a material can be converted into high quality useful products with standard techniques and equipment. For example, polyethylene, which is readily processed at low temperatures with no pre-treatments, would be considered more process-able than polyamide, which requires a much higher melting temperature and may need to be dried prior to processing.
Further, flexible materials or laminates that do not require further coatings for printability or printability additives during or post-extrusion are highly desirable for both quality and cost reduction. These features are quite attractive since polyethylene is a relatively inexpensive plastic currently in the order of approximately $1,500 USD per ton of unconverted resin.
Other key printability metrics include opacity, which is the ability of a material to stop the transmittance of light, quantified as the amount of light transmission. The opacity of a material is based upon the ratio of the diffused light reflectance of a material backed with a black body to the diffused reflectance of the same material backed with a white body. The higher the percent of opacity, the more opaque the material is said to be (ASTM D 589(b)).
Another key quality and printability standard is brightness. Brightness is a measure of light reflectance. Two objects may be described as “red,” however, the one that reflects the greatest amount of received wave length will appear to be brighter. When using paper specifications to describe reflectance of white light (all wavelengths), brightness is expressed on a scale of 0 to 100. Papers such as newsprint are typically about 55 bright. Most quality printing papers are in the order of 80 bright; the higher the brightness, the more brilliant the printed graphics. The brightness scale is arbitrary rather than expressing a percentage, hence papers can have a brightness level above 100.
Frequently, polymer-based films and sheets have favorable structural and other characteristics, however, because of surface characteristics, do not possess sufficient printability. A treatment to alter the surface of the plastic and other materials to make them more receptive to adhesives or printing inks may be necessary. This is known as “Corona Treatment.” Corona treatment includes a process of electrical discharges that create ozone, which in turn oxidizes the substrate surface and creates polar sites that contribute to strong bond formation. The treatment level is measured in dynes. A dyne in the (now deprecated) cgs system of units, is the force required to accelerate a mass of 1 gram by 1 centimeter per second squared. (1 dyne=1×105 Newton). Thus, in packaging, it is used as a measure of surface energy or polarity of a surface. The dyne level is an indicator of the ability to wet out the surface with a liquid, forming a chemical bond with an adhesive, coating, or ink. The dyne level of a surface typically needs to be 37 or higher, depending on the nature of the adhesive substance. (ASTM D 2578). Corona treatment achieving a specific dyne level and required printability is required for a broad range of flexible, semi-rigid plastic, polymer films, and sheeting within the industry. This is an expensive and time consuming process. Materials not requiring Corona Treatment often do not provide the proper combination of structural or cosmetic benefits based on performance specifications.
Other important flexible and semi-flexible film and sheet characteristics include the ability of packaging material or packages to resist or attenuate an electrostatic field such that the field's effects do not reach or influence the package's contents. A form of protective packaging that is used for solid state electronic devices to prevent damage caused by electrostatic discharges, electrostatic fields, and triboelectric charge generation, is commonly referred to as anti-static packaging, but more correctly called dissipative packaging. Often, dissipative packaging is considered very expensive and not considered to have advanced printability characteristics. Further, a stationary electric charge developed on a material as a result of an accumulation or deficiency of electrons in an area. All insulating materials are capable of developing and holding a static charge. Depending on the material, the tendency may be greater or smaller and may favor the positive or negative. Arrangement of the materials in a table according to their tendency to develop a charge, and the nature of the charge, is known as turboelectric series. The further apart two materials are in the series, the greater the tendency to generate and hold a charge when rubbed against each other.
For medical and other specialized applications, sterilization is often a required step in the manufacturing process. Therefore, materials must be used that are compatible with the process of sterilization. This performance metric is often referred to as “sterilize-ability”. This feature is defined as the ability to withstand contact with steam (moist heat) at 30 pounds pressure for 30 minutes, or contact with dry heat (circulating hot air) at 200 degrees Celsius for 15 minutes, or contact with ethylene oxide gas at specified temperature and pressure cycles. These processes would allow an article to be made free from living micro-organisms. Sterilizing agents may be steam, dry heat gamma rays, gas, or chemical sterilants.
The ability to withstand exposure to sun or other light can be an important material consideration. Light stability is the ability of a pigment, dye, or other colorant to retain its original color or physical properties when incorporated into plastics, inks, and other colored films or surfaces, when exposed to light. Additionally, the ability of a plastic or other material withstanding the deteriorating effect of exposure to sun or other light that results in physical material changes such as embrittlement, can be considered critical.
The weight, thickness, and density of materials are key considerations that materially affect cost, barrier characteristics, and yield of material substrates. These considerations greatly influence the film's structural performance and machine-ability. Normally, density is considered the mass of a given volume of material. In inch/pound units this is usually expressed in pounds per cubic foot. In ISO metric units, density can be given in kilograms per cubic meter (kg/m3) or grams per cubic meter (g/m3), although in packaging, grams per cubic centimeter (g/cm3) is more common. Relative density is the ratio of the density of the observed object to that of water (density of water is 1 gram per cubic centimeter. Relative density, being a ratio, is unitless. Material weight is another key factor influencing cost, yield, and thickness specifications. In packaging, the material weight is referred to as “basis weight” and generally refers to the mass of a given area of a material. In paper and films, the basis weight is the weight in pounds of a ream of paper cut to its basic size. The basis weight for most packaging papers is reported as the pounds weight of 3000 feet squared of paper. For paperboard and linerboard used for corrugated containers, basis weight is expressed in lb. per 1000 feet squared. In metric, this is reported as the grammage or the grams per meter squared of a given material. Often, the heavier the basis weight, the more strength in performance and barrier characteristics, however, since most packaging materials are sold by weight (most often by ton) the higher the basis weight, the higher to cost per thousand square inches (MSI) and the lower the area yield per dollar spent. Therefore, materials that contain a high basis weight, yet comparatively inexpensive when sold by weight, are very cost attractive packaging materials.
Also, environmental considerations are considered key. Minimizing energy use, green house gas emissions, water use, discharge, and maximizing recyclability and bio-degradability are considered very important. Packaging materials that contain mineral-based materials are considered environmentally superior to plastics, most particularly to oil-based carbon materials, synthetic resins, and polymers. Additionally, the elimination or reduction of the weight of packaging is a primary consideration effecting eco-friendly objectives. Reduction is the first priority in a program to improve the environmental performance of a packaging system. Some definitions of source reduction also include the elimination of toxic materials used in packaging. Source reduction is one of the four R's of environmentally responsible packaging; the other being reuse, recycle, and recover.
Methods of enclosing and sealing flexible film structures are important manufacturing considerations. The efficiency, speed of production, and performance of the closure directly impact the quality and performance of the packaging. The sealing surface is the surface to which the seal will be made or the surface of the finish of the container on which the closure forms the seal. Often, when sealing materials together, a “sealer” material must be applied to one or more of the sealed surfaces. This coating is designed to prevent or retard the passage of one substance through another. For example, highly porous substrates might have sealer coats applied to reduce the absorption of adhesives, printing inks, or subsequent coatings.
Within the packaging industry, several types of sealing methods are employed. The “L-Bar” sealer is a heat sealing device that seals a length of flat, folded film on the edge opposite the fold and simultaneously seals a strip across the width at 90 degrees from the edge seals. The article to be packaged is inserted into between the two layers of folded film prior to sealing. When it is desired to cut the continuous length of sealed compartments into individual packages, a heated wire or knife is incorporated between two sealing bars that form the bottom of the L. These bars then make the top of the seal of the filled bag and the bottom seal of the next bag to be filled. Dielectric sealing is a sealing process widely used for vinyl films and other thermoplastics with sufficient dielectric loss, in which two layers of film are heated by dielectric heating, and pressed together between applicator and platen electrodes. The films serve as the dielectric of the so-formed condenser. The applicator may be a pinpoint electrode as in “electronic sewing machines”, a wheel, a moving belt or a contoured blade. Frequencies employed range up to 200 MHz, but are usually 30 MHz or less to avoid interference problems.
Heat sealing is any method of creating as seal using heat. These include fusing plastic together by melting together at the interface or by activating a pre-applied heat-activated adhesive substance. Hot wire sealing is a sealing method using a hot wire to heat and fuse the plastic material. The sealing action simultaneously cuts through and separates the film. Impulse sealing is a heat sealing technique in which a surge of intense heat is momentarily applied to the area to be sealed, followed immediately by cooling. Solvent sealing is a method of bonding packaging materials, which depends of the use of small amounts of volatile organic liquid to soften the coating or surface of the material to the point where the materials will adhere. Ultrasonic sealing is the application of ultrasonic frequencies (20 to 40 kilohertz) to the materials being sealed together. The vibration at the interfaces generates enough localized heat to melt and fuse thermoplastic materials.
Several common methods of manufacturing flexible and semi-rigid sheets are found within the art. One such method is extrusion. This process forms thermoplastic film, or profile by forcing the polymer melt through a shaped die or orifice followed by immediate chilling. Profile extrusion produces continuous lengths of constant cross section.
Another method is cast extrusion. Using this method, film is made by extruding a thin curtain of thermoplastic melt onto a highly polished chilled drum. After the film solidifies, it is edge trimmed and wound into rolls for further processing.
Blown film is yet another, highly efficient method of manufacture. In this process, a thermoplastic film is produced by continuously inflating an extruded plastic tube by internal air pressure. The inflated film is cooled, collapsed, and subsequently wound into rolls. The tube is usually extruded vertically upward, and air is admitted through a passage in the center of the die as the molten tube emerges from the die. An air ring provides air flow around the outside of the bubble to increase initial cooling close to the die. Air is contained within the blown bubble by a pair of pinch rolls, which also serve to collapse and flatten the film. Film thickness is controlled by the die-lip opening, by varying bubble air pressure, and by the extrusion and take off rate. Thin films with considerable biaxial orientation can be produced by this method.
Films and sheets of different types, density, and thickness are often combined through lamination to accomplish the performance specification required for a package. Lamination is the process such that two or more sheets or films are adhesively bonded together in order to provide a group of enhanced properties not available in the individual films. During lamination, a “base film” is identified. The “base film” is an untreated film web stock to which print, coatings, laminations and other processes will be applied. Some lamination layers are oriented at right angles from other layers with respect to grain or strongest direction in tension, this technique is known as “cross lamination”. “Wet Lamination” joins two or more webs with aqueous or solvent based adhesives, which are driven off after joining. “Dry bond” laminating applies to adhesive to only one of the webs. After drying or curing, webs are joined with heat and or pressure. Other common laminating techniques are extrusion and hot melt in which the adhesive or bonding material is introduced in hot liquid form and the bond is affected when it solidifies. “Wax Lamination” is a laminate in which wax has been used to join two substrates. Wax is economical, however, at other than ambient temperatures, it can have poor performance properties.
Depending upon the material(s) used, additional manufacturing techniques may be required to enhance film performance. Often, this is the case or moisture or gas barrier requirements. One such technique is vacuum metalizing. It occurs upon the deposition, in a vacuum chamber, of vaporized aluminum molecules over the surface of a film or paper substrate. Metalizing provides a lustrous metallic appearance and when applied to plastic film, improves gas and light barrier properties. Metalized films are also used to dissipate static electrical charges, reflect radiant heat and for microwavable packaging. Adding Nitrile resin is another polymer material option containing high concentrations of nitrile having outstanding barrier properties. Generally the constituents are greater than 60% acrylonitrile along with comonomers such as acrylates, methacrylates, butadiene, and styrene.
Various films are used in multi-layered laminated structures to achieve the desired results. Polyethylene film is by far the largest volume transparent flexible packaging material because of a combination of transparency (low density types), toughness, heat seal-ability, low water vapor transmission rate, low temperature performance and low cost. Polyethylene films are highly permeable to oxygen and other non polar gases and have high viscoelastic flow properties. Available with a wide range of specific properties to meet individual needs. PE can be clear or translucent depending on density. It is a tough, waxy solid, that is unaffected by water and is inert to a large range of chemicals. Polyethylene is marketed in three general categories: low, medium, and high density. Films can also be made of polylactic acid (PLA), which is a biodegradable polymer made from renewable resources (primarily corn derived dextrose). Only recently made available in commercial quantities, PLA has potential applications in wraps, films, and thermoformed parts. Polyethylene terephthalate (PET) film is a thermoplastic film of high strength, stiffness, transparency, abrasion resistance, toughness, high temperature resistance, and moderate permeability. Generally used in sections of 0.0005 inch or less and laminated to less expensive materials. PET's high temperature tolerance makes it a preferred material for ovenable applications. PET is often referred to as polyester. While this term is not incorrect, polyester is a family name for a large group of polymeric materials. PET refers specifically to the polyester used in packaging applications. Polyvinyl alcohol (PVAL) is a water-soluble thermoplastic prepared by partial or completed hydrolysis of polyvinyl acetate with methanol or water. Its principal uses are in packaging films, adhesive, coatings, and emulsifying agents. Its packaging films are impervious to oils, fats, and waxed, have very low oxygen transmission rates, and most often used with other thermoplastics as a barrier coating or layer. PVAL coatings and layers must be protected from water.
Polypropylene (PP) film is a transparent, tough, thermoplastic film usually made by cast extrusion. Un-oriented film is soft and becomes brittle at low temperature, however this property as well as strength, stiffness, and clarity can be improved by orientation e.g. bi-axially oriented polypropylene (BOPP). Polystyrene film is a transparent, stiff film of high permeability and moderate temperature resistance, typically made by extrusion or casting, and can be oriented to improve strength. PVC film is a transparent to translucent film (depending upon plasticizers and stabilizers) made by extrusion or casting. Excellent grease and solvent resistance, low to moderate gas permeability, moderate temperature range. Films can also be made of polyamide (PA). Commonly known as nylon. A polymer made by the reaction of a dibasic acid and an amine. There are many dibasic acids and many amines, giving the possibility of many polyamides, few of which are used in packaging. PA is used almost entirely as a film or sheet material in packaging applications. The clear film offers a good oxygen barrier, is particularly tough and abrasion resistant, and can be drawn easily into thermoformed trays. However, it is a poor moisture barrier, does not heat seal, and has cost disadvantages. Films can also be made of polychlorotrifluoroethylene (PCTFE or CTFE), which is a plastic material characterized by exceptional moisture and good oxygen barrier characteristics as well as good clarity and easy thermoformability. Its costs restrict it mostly to the pharmaceutical industry. Films can also be made of polyester, a polymer made by the reaction of a dibasic acids and many glycols, giving the possibility of many polyesters, some of which are thermosets and some of which are thermoplastics. Packaging uses a thermoplastic polyester made by the reaction of terephthalic acid and ethylene glycol. The term polyester commonly refers to poly(ethylene terephthalate), abbreviated most commonly as PET. It is also known as PETE on plastic identification codes. Metalized polyester film is a PET film on which a minute amount of aluminum has been vacuum deposited to improve barrier properties, enhance appearance or to produce a heating structure for microwave packaging applications. Films formed of kraft and other papers are fiber roll stock and sheet paper materials are used in flexible film applications for low cost layers providing structure, stiffness, dead-fold, tensile strength and some degree of printability.
A problem that exists with prior packaging products and films is that these products may not incorporate environmentally friendly materials and designs, particularly with laminated structures and most particularly at low cost levels that offer affordability. Environmentally friendly materials can have desirable attributes such as biodegradability, compostability, a high recycled content, recycle-ability, and may also use less energy, pollute less, and generate fewer greenhouse gases in their manufacture than previous materials. Such environmentally friendly materials are increasingly in demand from consumers and retailers, and can be beneficial for manufacturers by reducing adverse environmental impact of the material.
Another significant problem that exists with prior flexible film packaging, laminations, and composites is the high concentration of expensive plastic and polymers required to achieve the performance specifications needed. Another problem is the need for laminating very expensive combinations of plastics, foils, coatings, metalized films, etc to achieve structural, barrier, sealing and printability aspects; this is the most significant problem within the art as polymer based materials can range from approximately $1,500 to $4,000 per ton of pre-converted resins, depending upon the material(s) used and the application. Additional problems include obtaining bright, white, opaque printing surfaces on barrier films without multi-layer laminations, corona treating for ink adhesion, or coating that treat film surfaces for quality lithography, flexographic, and offset printing. Other desired characteristics include sterilize-ability, anti-static/dissipative characteristics, and machine-ability during converting and printing.